None really, considering the price. Only possible one is the plastics used to build it, but at this price those criticisms are both churlish and missing the point.

some of you are thinking - this is a cheap kit lens, it has to be garbage. Well, you couldn't be more wrong.

The bottom line :

* This lens produces very good images at a very low price. *
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Forget the Canon kit lenses that came before. This lens produces clear, crisp images that are entirely acceptable for most purposes. They may not have the zippy colours that you see in L glass or primes, but for a zoom lens of this price, this optic can produce results that are a huge leap forward from its predecessors.

As a beginners lens or a lens to go roughing it with, where you don't want to take expensive or heavy glass, then this lens is *highly recommended*.

The design of this lens dates back to the Canon FD mount and the 1980's. But don't let that put you off. This is a quality optic, that will produce very fine results. The images will have that 'pop' to them that quality optics produce.
No USM, but even so the autofocus is not that noisy. Your subject will probably not hear it. The autofocus accuracy if pretty good too. You won't have many problems with that.
As a group photo lens on a full frame camera or a standard lens on a 1.6x sensor, this lens is a very usefull tool to carry, and it won't take up much space or weight in your bag either.
If you are on a limited budget and you want the best possible quality pictures, I would highly recommend this lens.

Very solid contruction. Smaller than equivalent full frame lens. Very good image quality. Silent autofocus. Comes with hood and case.

Cons:

Quality control poor.

I have this lens in the Canon fit.
This lenses autofocus motor broke twice on me, very early on. The first time within maybe the first 500 focuses, the next within the next 200 focuses. I was very dissapointed by this. Both times I sent it to Sigma repair (postage paid by myself to them), the lens was returned promply by them. Whether or not I was unlucky buying a bad copy is immaterial - I spent my money to take photos, not have all the hassle of posting my lens back and forth. I won't be buying a Sigma product again - it's Canon (or maybe Tokina) for me from now on.
A shame really, as the build materials are professional grade, the optics are very good quality. It comes with hood and pouch. It should be a winner.
But at the end of the day for me it was let down by very poor quality control at Sigma. I would expect them them to be testing each EX lens thoroughly before being sent out, as they claim these are professional tools. Either they are not doing this or there is some flaw in the design or materials of the lens.